The present invention relates to an attachment in the nature of a clip for securing a resonator crystal to a circuit board and particularly relates to a clip for surface-mounting a coupled-dual resonator crystal on a circuit board with appropriate electromagnetic shielding.
A typical coupled-dual resonator crystal includes a plate of piezoelectric material having input, output and common electrodes with a first resonator formed by the input electrode and common electrode and a second resonator formed by the output electrode and common electrode. Such coupled-dual resonator crystals are used extensively in electronic applications including IF filters, for example, for mobile and cellular radios.
In earlier work, it was found that many of the two-pole monolithic crystal filters comprised of coupled-dual resonator crystals used in mobile radios did not meet their required ultimate stopband rejection attenuation because of the close proximity of the input and output terminals of the crystal filters in a predetermined frequency range (8 MHz-25 MHz). The problem was solved by providing a grounded metal shield attached to the fixture pin into which the center pin of the crystal would be inserted, thereby placing the input terminal of the filter in one grounded conductive cavity and the output end of the filter in another grounded conductive cavity. Later, a linear array of plated-through holes on a printed circuit board was used in conjunction with the terminals of a two-pole filter so that a straight line through each of the three terminal pins was perpendicular to the axis of the array of plated-through holes. All the plated-through holes were attached to one another and grounded. The center pin of the crystal was inserted into and soldered to one of the holes toward the center of the array. This placed the input of the filter on one side of the array and the output of the filter on the other side, thereby providing a grounded shield between the input and output of the filter.
Subsequently, a two-pole crystal filter (a single coupled-dual crystal) in the 45 MHz range as one of the IF filters in a radio was tested, revealing a major problem with respect to the ultimate selectivity achieved by the two-pole crystal on the actual circuit board. The problem was exacerbated because of the close spacing between the input and output terminals. However, the problem was solved by adding a chip capacitor on the opposite side of the board on each side of the crystal along the hole array with both terminals of each capacitor attached to a hole and one terminal of each cap attached to a hole immediately adjacent to the crystal center pin.
The present invention seeks to provide similar results with a surface-mounted crystal. To accomplish this in accordance with the present invention, there is provided a clip for receiving the crystal housing. The crystal projects from an open end of the clip body and has in an aligned array input and output terminals with a common terminal therebetween. The terminals project endwise from the crystal housing in a generally Z-shaped configuration whereby the linear tips of the Z-shaped configuration project beyond the margins of the clip body for resting on contacts on the printed circuit board. The clip body includes a pair of pads at its opposite end likewise for resting on contacts with the circuit board. Consequently, the clip and crystal contained therein are mechanically balanced on the circuit board. Because the clip body is generally complementary in configuration to the crystal housing and hence in good electrical contact therewith, the pads afford two additional ground contacts for the crystal itself, as well as a ground contact at the common terminal.
Cantilevered from the body of the clip by a resilient tab extending from one side of the clip body adjacent an open end thereof is a ground shield for shielding the output terminal from the input terminal. The shield has a first shield portion which extends from its proximal edge at its connection with the tab to a distal edge projecting from the opposite side of the clip body. The first shield portion extends from the clip body in a plane parallel to a plane containing the generally Z-shaped common terminal and, when the crystal is disposed in the attachment, the first shield portion engages the common terminal. A second shield portion extends generally at right angles to the first shield portion and overlies the common and output terminals. The second shield portion specifically shields the output terminal from that portion of the electromagnetic field radiating from the part of the input terminal from the point it leaves the crystal housing to the first bend in the input terminal. The first shield portion shields the output terminal from the electromagnetic wave propagating from that part of the input lead from the first bend to the second bend. Preferably, the end edge of the first shield portion extends beyond the end or tip of the common terminal.
With the crystal within the clip, the package is disposed on a circuit board with the pads and terminals in electrical contact with correspondingly located contacts on the circuit board. While the clip will inherently make electrical contact with the crystal housing, preferably a solder attachment of the clip to the crystal housing affords better ground contact than a strictly pressure contact. The configuration of the first and second shield portions makes the shield intrinsically stable mechanically without attaching it directly to the center pin of the crystal. Also, the clip and crystal are supported by the circuit board at five discrete locations, affording improved resistance to shock and vibration and greater stability. Furthermore, for a given clip size, longer or shorter crystal packages may be provided, although each clip can be sized for a particular size of crystal.
In a preferred embodiment according to the present invention, an apparatus is provided for surface-mounting a coupled-dual resonator crystal to a circuit board, the crystal having a generally elongated housing with input and output terminals and a common terminal therebetween, all of the terminals projecting from an end of the crystal. The apparatus comprises a clip including an elongated body forming a receptacle for receiving the crystal housing, the clip body having an open end, and an electromagnetic radiation shield including generally angularly related first and second shield portions. Resilient means engageable with the crystal for retaining the crystal within the body, and means connecting the body and the shield to one another and spacing the shield from the body adjacent the open end of the body are provided. The first shield portion comprises a first generally planar element extending across the open end for disposition between the input or output terminal and the common terminal of the crystal when the crystal is disposed in the clip.
In a further preferred embodiment according to the present invention, there is provided apparatus for surface-mounting a coupled-dual resonator crystal to a circuit board, the crystal having a generally elongated housing with input and output terminals and a common terminal therebetween, all of the terminals projecting from an end of the crystal. The apparatus comprises a clip including an elongated body forming a receptacle for receiving the housing of the crystal, the clip having an open end, an electromagnetic radiation shield including a shield portion and means connecting the body and the shield portion spacing the shield portion from the body and extending from the clip body adjacent an open end. The shield portion comprises a generally planar element extending across that open end for disposition between the input or output terminal and the common terminal of the crystal when the crystal is disposed in the clip.
Accordingly, it is a primary object of the present invention to provide an attachment for surface-mounting a coupled-dual resonator crystal affording improved mechanical and electrical contact with the circuit board, as well as shielding between the input and output terminals.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.